1. Field of the Invention
This invention relates to low dielectric constant films useful in the production of integrated circuits.
2. Description of the Prior Art
Certain organic polymer dielectric films have important applications in integrated circuit (IC) fabrication. Such polymers are used as electrical insulating layers for multilevel interconnect structures in advanced IC devices. These materials are attractive because their dielectric constants are lower than that of the standard IC interconnect insulator, silicon dioxide (glass), which has a dielectric constant of about 4.0. It is well known that faster signal processing, lower power consumption, and reduced noise in circuitry results from use of selected organic polymers as the dielectric material, particularly with those which possess dielectric constants below 4.0, and especially those which have a dielectric constant below 3.0.
Production of interconnect structures in IC devices typically require the deposition of metal (usually aluminum or copper) to form wiring patterns that connect the circuit transistors. These deposition processes occur when the substrate is maintained at temperatures between about 350.degree. C. and 500.degree. C. Thus, the electrical insulating material must be chemically and mechanically stable at the metal deposition temperature.
Many organic polymer insulating films being considered for this application are chemically and mechanically unstable above 400.degree. C. or even 350.degree. C. Many such materials have glass transitions temperatures (Tg) below 400.degree. C. or even below 300.degree. C. When a polymer transcends from a glassy state to a rubbery state, it passes through the glass transition temperature or Tg. Polymers generally have less mechanical strength compared to aluminum or copper. Polymers also typically have a higher thermal expansion coefficient (TEC) compared to aluminum or copper. When a polymer transcends from the glassy state to the rubbery state, its mechanical strength or modulus decreases substantially. In this application, an IC interconnect structure would be made of aluminum or copper wiring patterns and polymer insulator layers. In view of the differences in TEC, a polymer would expand dimensionally to a greater extent than the wiring as the substrate temperature rises from room temperature to 350.degree. C. and above. This mismatch in TEC causes large mechanical stresses to develop which may cause severe physical damage to the metal interconnect structure. Metal lines may break or become dislocated from their original positions. In addition to thermal-mechanical stability, the insulating layer must not thermally decompose at the same high processing temperatures; for example, thermal decomposition may lead to loss of mass through evolution of volatile fragments from the polymer.
Furthermore, the film must be impervious to photoresist processing steps. Such processes involve exposing the insulating layer to aggressive liquid chemicals for photoresist development and stripping. These solvents can either swell the polymer insulating layer or dissolve it off the substrate.
In standard processing of polymer insulating layers, the layer is cured at 400.degree. C. or higher, typically between 400-500.degree. C., for at least 30 minutes, but more typically 60 minutes. In this curing process the polymer undergoes chemical reactions, such as crosslinking, which render the material strong and glassy, and impervious to common organic solvents. These are desirable properties for a insulating layer. However, the time and temperature conditions needed to achieve these characteristics may cause damage to the aluminum or copper wiring. While aluminum or copper wiring may be safely exposed to 400-500.degree. C. conditions for brief periods (up to 5 minutes as in metal deposition), the metal may be damaged if the time is as long as 30-60 minutes. This damage is characterized as stress-induced voiding, in which thermal mechanical stresses lead to thinning of the aluminum wires in localized areas. Thus, it is desirable to minimize the time required to complete the deposition and formation of insulating layers.
The present invention solves this problem by providing a method for curing polymer insulating layers for IC interconnect fabrication. The method for curing comprises exposure of the polymer layer to electron beam radiation. The electron beam tool is designed and used to expose all parts of the film to a uniform flux of electrons. This tool is also known as a wide area electron beam tool. The electron beam exposure causes chemical reactions to occur in the polymer structure which cause the formation of crosslinks between polymer chains. The crosslinks lead to higher mechanical strength and higher Tg, lower TEC, greater thermal-chemical stability, and greater resistance to aggressive organic solvents.